Lithium ion secondary battery has lighter weight and larger capacity as compared with conventional lead secondary battery, nickel-cadmium secondary battery and so forth, and has widely been used as a power source for electronic devices such as mobile phone, notebook type personal computer and so forth. It has recently been used also as batteries for electric vehicle, plug-in hybrid car, pedelec and so forth.
The lithium ion secondary battery is basically composed of a cathode, an anode, an electrolyte, and a separator. As the anode, there has been used carbon, lithium titanate and so forth, which allow intercalation and deintercalation of metallic lithium or lithium ion. Meanwhile, a lithium salt and organic solvent or ionic liquid capable of dissolving the lithium salt have been used as the electrolyte. The separator is a component placed between the cathode and the anode so as to keep electrical isolation between the two while allowing the electrolyte to pass through the pores thereof, and is configured by using porous organic resin, glass fiber or the like.
The cathode (also referred to as “cathode layer”, hereinafter) is generally configured by an active material which allows intercalation and deintercalation of lithium ion, an electrically conductive auxiliary which ensures an electric conduction path (electron conduction path) to a current collector, and a binder which binds the active material and the electrically conductive auxiliary.
The electrically conductive auxiliary is typically configured by using a carbon material such as acetylene black, carbon black, graphite or the like.
Examples of the active material generally used include metal oxides composed of lithium and transition metal(s) such as LiCoO2, LiNiO2, LiNi0.8Cu0.2O2 and LiMn2O4, and other known examples include LiMPO4, Li2MSiO4, LiMBO3 and derivatives obtained from these basic structures by element substitution or compositional change (simply referred to as “derivatives”, hereinafter). Now, M mainly contains transition metal element (s) characterized by variable valency, such as Fe, Mn, Ni and Co.
The metal oxide is generally low in electron conductivity. Efforts have therefore been made on improving the electron conductivity of the cathode which uses the metal oxide as the active material, typically by mixing the metal oxide with the above-described electrically conductive auxiliary or by providing a carbon coating or by allowing carbon grain, carbon fiber or the like to adhere on the surface of the metal oxide (Patent Literatures 1 to 6 and Non-Patent Literature 1). In particular, the carbon coating on the surface of the metal oxide has been considered to be effective for the purpose of obtaining excellent battery characteristics.
Among the oxides described above, Li2MSiO4 represented by iron lithium silicate and manganese lithium silicate and derivatives thereof (they may generally be referred to as “lithium silicate” on occasions) are expected for their theoretically large capacity since they contain two lithium ions in the compositional formulae (Patent Literatures 7 to 9, Non-Patent Literature 2).